The present invention relates generally to energy delivery systems, and more particularly, to a system and method for testing winding and winding connection displacements in a transformer.
Electric utilities and other organizations are responsible for supplying an economic, reliable and safe source of electricity. Three major components are employed in an energy delivery system to provide the electricity to the end user, the generator, the transmission line and the transformer.
Generators are rotating machines operated in a manner such that electricity is created when mechanical energy is used to rotate the generator shaft. A generator rotor is coupled to the shaft, and when the shaft is rotated, thereby rotating the rotor, a voltage and current is caused in the generator stator. One typical form of mechanical energy used to generate electricity is steam, which is passed through a turbine that forces the generator shaft to rotate. Steam is often created by boiling water using coal, natural gas or nuclear fission heat sources. Or, steam may be taken directly from naturally occurring geothermal sources. Other sources of mechanical power employed for rotating a generator rotor may also include hydroelectric power or wind power. Since the end user of the electricity is rarely located near a generator, the electricity generated by the generator must be xe2x80x9ctransportedxe2x80x9d to the end user.
The second major component employed in an energy delivery system is the transmission line. Transmission lines consist of a grouping of wires which connect the generator to the end user. The xe2x80x9camountxe2x80x9d of electricity that a transmission line can carry depends primarily upon the diameter of and number of the conductors (wires) used on the transmission line, and the voltage at which the transmission line is operated at. Typically, transmission lines from the generators employ a relatively high voltage so that a large amount of electricity is economically and reliably transported over long distances to locations where large concentrations of end users are found, such as a city or a large industrial manufacturing plant. Examples of extra high voltage (EHV) and intermediate transmission voltages employed in the industry include, but are not limited to, 500 kilo-volts (kV), 230 kV, 138 kV, 115 kV, 69 kV and 46 kV. Typically, lower transmission line voltages are employed on the transmission line distribution system (such as, but not limited to, 25 kV, 20 kV, 13.8 kV, 12 kV, 4 kV, 480 V and 240 V) to provide energy to the end user""s premises connection point.
The third major component employed in an energy delivery system is the transformer. The transformer is a device that changes voltage. Generally, voltage from the generator is a lower voltage than used by the transmission lines that transmit the electricity to the end user. Furthermore, the voltage used by the end user is much lower than voltage than used by the transmission lines. Thus, the transformer couples elements of an energy delivery system that employ different voltages.
For example, two voltages typically found in a home are 240 volts and 120 volts. An EHV 500 kV transmission line may be delivering power to a city that employs a 230 kV transmission line system to deliver energy to a 13.8 kV distribution system. A 500/230 kV transformer changes voltage from 500 kV to 230 kV, thereby allowing two transmission lines having different operating voltages (500 kV and 230 kV) to be coupled together. Such a transformer has at least two terminals, a 500 kV terminal and a 230 kV terminal. Similarly, a 13.8 kV/240V/120V transformer may be used to convert voltage of the 13.8 kV distribution system to a voltage used in the end user""s home or office. Thus, transformers allow the various voltage generators, transmission lines and distribution lines to be coupled to a home, office or other facility where the end user will be using the electricity.
Transformers come in many different sizes, shapes and constructions. Typically, transformer size (rating) is specified as the product of the maximum voltage and current, as measured from one side of the transformer, that the transformer is capable of converting at a particular operating condition. Such operating conditions include temperature and/or altitude. For example, a 500/230 kV transformer may be rated at 300 MVA (3,000 kilo-volt-amps) when operating at sea level and at 65xc2x0 Celsius rise above ambient. Transformers may be constructed as separately insulated winding transformers or auto transformers, and as single phase or multiple phase transformers. The operating voltages, ratings and winding types of transformers employed in the industry, well known to one skilled in the art, are too numerous to describe in detail here other than to the extent necessary to understand the present deficiencies in the prior art.
All transformers, independent of size, rating and operating voltage, have several common characteristics. First, the transformer is constructed from one or more windings, each winding having a plurality of individual coils arranged and connected in an end-to-end fashion. In some transformers, the winding is made by wrapping a wire around a laminated solid member, called a core. Alternatively, there may be no core. However, in all transformers, the individual windings must be electrically isolated from each other. An insulation material is wrapped around the wires such that when the plurality of coils are made, the metal wires of each winding are physically and electrically separated, or insulated, from each other. Insulation materials wrapped around the windings may vary. Paper, impregnated with oil, is often used. Other types of transformers may use only paper, or may use another suitable material such as a polymeric compound.
Maintaining the electrical insulation between the windings is absolutely essential for the proper operation of a transformer. In the event that the electrical insulation is breached, such that electricity passes from one winding coil across the breach to another winding coil, special protective devices will operate to disconnect the transformer from the electrical system. The devices, by removing electricity applied to the transformer, interrupt the undesirable current flow through the insulation breach to minimize damage to the transformer. This condition is commonly referred to in the industry as a transformer fault.
Transformer faults are undesirable for at least two major reasons. First, end users may become separated from the energy delivery system, thereby loosing their electrical service. Second, transformer faults may result in large magnitudes of current flow, known as fault current, across the breach and through the transformer windings. Also, faults occurring on the energy delivery system at locations relatively close to the transformer may result in large fault currents flowing through the transformer. Often, fault current may be orders of magnitude greater than the highest level of normal operating current that the transformer was designed to carry. Such fault currents may cause severe physical damage to the transformer. For example, a fault current may physically bend portions of the transformer winding (winding deformation) and/or move the windings out of their original position in the transformer (winding displacement). Such winding deformation and/or displacement can cause over-voltage stresses on portions of the winding insulation and exacerbate the process of the naturally occurring deterioration of the winding insulation that occurs over a period of time. The fault current may further increase damage to the insulation, or damage insulation of adjacent windings, thereby increasing the magnitude and severity of the fault. In the most extreme cases, the fault current may cause an ignition in the transformer oil, resulting in a breach of the transformer casing and a subsequent fire or explosion.
Therefore, it is desirable to ensure the integrity of the transformer winding insulation. Once a transformer fault occurs, it is usually too late to minimize transformer damage and to reduce the period of electrical outage. The electric utility industry takes a variety of precautionary steps to ensure the integrity of winding insulation in transformers. One important precautionary step includes periodic testing of the transformer. Various tests are used to predict a probability of a future fault. One test commonly employed in the industry to detect winding deformation and/or displacement is the low voltage impulse test.
To perform a low voltage impulse test according to prior art methods, an electrical pulse or signal is applied to one terminal of a transformer. That is, a signal or pulse is applied to the input winding of the transformer. The signal or pulse on the output side of the transformer (output winding) is then measured. The input and output signals or pulses are analyzed using a variety of techniques. One analysis technique is to perform a frequency response analysis (FRA) which measures one characteristic of the input and output signals over a predetermined frequency range. One commonly employed technique is to process the measured input and output signals or pulses by applying a fast Fourier transform (FFT) to the signals. The FFT of the output signal is divided by the FFT of the input signal and the resultant admittance, as a function of frequency, may be plotted for the transformer. The input signal or input pulse may be applied to the high voltage, low voltage, neutral or other suitable terminal that is available on the transformer. The output signal is taken from another selected terminal on the transformer. For example, a low voltage pulse is applied to the high voltage terminal of the transformer winding and the output pulse is measured on the low voltage terminal of the transformer. Such a test is commonly known as a low voltage impulse test because the voltage of the applied input signal or pulse is much less than the impulse voltages used to test for dielectric integrity in a high voltage laboratory or at the transformer manufacturing site. When a series of identical pulses or signals are applied to the transformer winding, in accordance with prior art transformer testing procedures, and the resultant measurements are averaged together, the resultant plot is often referred to as the transfer function or characteristic signature of the transformer winding configuration being tested.
In a static situation, a test engineer could reasonably expect that the characteristic signature of the transformer winding would not significantly change with time. For example, the test engineer could reasonably expect that a transformer winding tested one year after being placed in service, assuming that nothing has changed within the transformer during that year, could be tested and have output measurements that would substantially match the output measurements taken a year earlier.
However, static conditions rarely occur in the field. Each time current flow is adjusted in the transformer, mechanical stress in the windings change. Abrupt changes in current flow can occur every time a portion of the electrical transmission system is reconfigured by switching, or every time lightning strikes the transmission system nearby the transformer. Many other events may also cause abrupt changes in current through the transformer on a regular and frequent basis. This is a basic reality of the operation of the electric system. Transformer windings are designed to accommodate a number of reasonable magnitudes of abrupt current change over the operating life of the transformer. Yet, abrupt current changes in excess of the design limits are occasionally encountered. When these conditions occur, the windings may permanently bend from their original position, hereinafter referred to as winding deformation. Or, the windings may move slightly from their originally installed position, hereinafter referred to as winding displacement. Winding deformation and displacement may stress, crack and/or otherwise impair the insulation around the windings. Furthermore, the impairment caused to the winding insulation by each abrupt change in current is cumulative. That is, the damage is not self-repairing or healing. Eventually, the damage may become sufficient to cause a breach in the insulation. Then, a fault will occur and the transformer will become damaged, thereby requiring the transformer to be taken out of service for repair or retirement.
Additionally, winding deformation and/or displacement may alter the voltage gradient around the bent portion of the coils. If the winding deformation and/or displacement decreases the gap between two adjacent winding coils, the voltage gradient may become more concentrated around the bend. The increase in the voltage gradient may be sufficient, over a short or long period of time, to breach the insulation, thereby causing a fault. Or, the increased voltage gradient may cause a temperature increase around the deformed and/or displaced portion of the coils. The increased temperature increases the rate of degradation of the winding insulation. In an oil filled transformer, the temperature increase may alter the properties of the transformer oil, and possibly result in electrical partial discharge which in turn results in the formation of undesirable gasses.
Thus, periodic testing is performed to determine and/or estimate the amount of cumulative damage to the transformer resulting from the normal (and abnormal) day-to-day operating conditions that the transformer has been subjected to. If the tests indicate potential problems, the transformer can be scheduled for maintenance, or replaced if necessary, in a timely and controlled manner that results in the least disruption in service to the end users. Furthermore, transformers are very expensive pieces of equipment, thus repairing a transformer before permanent damage occurs is desirable.
Prior art low voltage impulse tests present many unique problems. One significant problem is that a precise, repeatable input testing signal or pulse of known energy content to be sufficient for the test must be applied to the input terminal of the tested transformer winding when prior art frequency response analysis techniques are used to measure the frequency response of the transformer winding. If the applied input test signals/pulses are not identical to each other, the resultant characteristic signature of the tested transformer windings will not be accurate. For example, the prior art has no objective test accuracy or bandwidth limit analysis, so an unknown pulse at the input will compromise the test result without detection. In addition, the time delay between pulse applications for the prior art should be constant to prevent random distortion of the input pulse which affects the characteristic signature. For example, if the pulse intervals are not constant, the energy storage remaining in the transformer winding configuration will be different between pulses, thus altering the load impedance of the transformer and therefore, changing the parameters (frequency energy content) of the applied pulse. Furthermore, test signal/pulse generators or test pulse generators capable of providing such exact and repetitive input signals or pulses are expensive.
Therefore, it is desirable to have a valid and reliable low voltage impulse testing system and method that does not require a plurality of identical input test signals/pulses for a single test. Also, it would be desirable for the test equipment to be inexpensive, to be easily portable, and to be easily implemented in the field where the transformer is located. Furthermore, it would be desirable to have the test signal/pulse generator configured to provide a wide variety of test signals/pulses suitable for testing a wide variety of transformers.
The present invention provides a system and method for determining a characteristic signature of a winding residing in a device. Briefly described, in architecture, the system and method can be implemented as follows.
The person conducting the test of the winding, hereinafter referred to as the testing personnel, prompts a pulse/signal generator to generate a pulse or signal that is applied to the winding which is to be tested. A sensor detects an output pulse or signal after the applied input pulse or signal has propagated through the winding. Data corresponding to the applied input pulse or signal, and data corresponding to the detected output pulse or signal, is stored in a memory. The testing personnel prompts the pulse/signal generator to apply a suitable number of additional pulses or signals as described above such that a data base of pairs of input and output pulses or signals are accumulated in the memory. For each of the subsequent applied input pulses or signals, the testing personnel may optionally actuate a pulse/signal width adjuster, and/or actuate a pulse/signal voltage adjuster, such that the nature (width and/or voltage) of the subsequently applied input pulses or signals are varied.
After a suitable number of input pulses or signals have been applied to the winding, the testing personnel prompts a processor to retrieve the stored data and to calculate a characteristic signature of the tested winding. The processor calculates the characteristic signature of the winding by executing logic employing a unique computational method. First, the auto-spectral density (Gxx) is calculated. Gxx is defined by the complex conjugate of the fast Fourier transform (FFT) of the input pulse or signal times the FFT of the same impulse or signal. Second, the cross-spectral density (Gxy) is calculated. Gxy is defined by the complex conjugate of the FFT of the input pulse times the FFT of the output pulse. The logic then calculates the characteristic signature [H(f)] for the winding such that H(f) equals the average of the average of the Gxy""s divided by the average of the Gxx""s for the respective pairs of input and output pulses or signals. This H(f) is representation is chosen for noise rejection in the output signal, since the output signal is much smaller than the input signal. For example, only the output components which are correlated with the input pulse are accepted. Other components, such as, but not limited to, un-correlated output noise, are rejected by definition. Thus, a good signal to noise ratio for a relatively small signal in a relatively noisy environment is provided. In addition, equations using spectral densities (Gxx, Gxy and Gyy) can be determined when input pulses or signals that are slightly different, or even very different, from each other.
To verify the accuracy and validity of the calculated characteristic signature H(f) of the winding, a coherence function xcex32xy(f) is calculated, according to the equation below:
xcex32xy(f)=|Gxy(f)2/Gxx(f)Gyy(f)
The coherence function xcex32xy(f) is a real valued function having a magnitude ranging from 0 to 1. A value of 1 would indicate a perfect linear relationship from the input pulse or signal to the detected output pulse or signal. A value of 0 would indicate a complete non-linear relationship between the input and detected output pulses or signals, or that there was not sufficient input energy in the applied pulse or signal to transfer sufficiently to the output. The coherence function is also sensitive to alaising on the input and output digital records. The coherence function will also indicate low numbers for high input pulse noise levels.
Additionally, a random error function Er|H(f)| is calculated, according to the equation below:
Er|H(f)|=[1xe2x88x92xcex32xy(f)]xc2xd/|xcex3xy(f)|(2nd)xc2xd
The random error, Er|H(f)|, provides a statistical analysis of the test data and defines a 95% confidence interval for the test data that is graphed over a frequency range of interest. Such a graph plots two lines, with the spacing between the two lines indicating the 95% confidence interval for the test data. The error function indicates how well the test is performed from a first testing of the transformer winding and a second test on the same winding. For example, the value of the error function will increase significantly if there is a problem with the connections or the leads in one of the tests. The error function will not increase significantly for a change due to transformer winding deformation and/or displacement.
One embodiment of the winding testing unit includes logic that analyzes the coherence function and the random error function. Based upon pre-defined criteria, the comparison numbers may be displayed with a color coded system. For example, if the comparison displays a number in green, then little or no change has occurred between the two compared tests of the same transformer winding. If the comparison numbers are displayed in yellow, then the yellow color indicates that some change has occurred. However, such changes denoted by the yellow color may be considered to be associated with temperature differences between the test, differing conditions of transformer oil between the tests, or normal aging of the insulation that occurs over a number of years. If the comparison numbers are displayed in red, an indication of a significant change in the tested winding is indicated. Should the comparison numbers be well into the red zone, the tested winding should be inspected and scheduled for possible repair in the near future. Thus, this embodiment of the winding testing unit is particularly well-suited for use by an individual who is not necessarily skilled in the art of analyzing transformer test results, such a technician or other maintenance personnel.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.